The present invention relates to an electrically-driven disc brake for generating a braking force by a rotational force of an electric motor.
Among such disc brakes, There is an electric disc brake in which a caliper body is floatingly supported by a carrier secured to a non-rotating portion of a vehicle, and the caliper body is provided with a piston opposed to one of a pair of brake pads supported by the carrier on both sides of a disc rotor, an electric motor, and a motion converting mechanism for converting rotation of a rotor of the electric motor into a linear movement and for transmitting such movement to the piston. In such an electric disc brake, when an operator depresses a brake pedal, a force acting on the brake pedal (or displacement amount) is detected by a sensor, and the rotation of the electric motor is controlled by a controller on the basis of a detection result to obtain a desired braking force.
A ball-screw mechanism or a ball-ramp mechanism has been mainly used as the motion converting mechanism for converting the rotation of a rotor of an electric motor into the linear movement and for transmitting such movement to a piston since they can provide large thrust or braking force. However in conventional electric disc brakes, There has been no special speed reduction mechanism between the electric motor and the motion converting mechanism. Therefore, motor torque, thence, a motor body is made large and thus the caliper body itself also made bulky, thereby worsening installation capability onto the vehicle.
In some cases, the speed of the rotation of a motor is decreased by using a planetary gear and the rotation is converted into a linear movement by a precise roller-screw mechanism to generate a thrust force thereby to obtain the braking force. However, since a great speed reduction ratio cannot be achieved by the planetary gear, There is limitation in compactness of the electric motor, and, thus, the fundamental solution cannot be achieved.
Further, in some cases, the speed of the rotation of a motor is decreased by using a worm gear and the rotation is converted into a linear movement by a precise roller-screw mechanism to generate a thrust force thereby to obtain the braking force. In this case, however, since the precise roller-screw mechanism is located perpendicular to the worm gear, the interior of the caliper body cannot be utilized efficiently, with the result that There is limitation in compactness of the caliper body, and, thus, the fundamental solution cannot be achieved.
The present invention is made in consideration of the above-mentioned drawbacks, and an object of the present invention is to provide an electric disc brake in which motor torque can be reduced by incorporating a speed reduction mechanism capable of a great speed reduction ratio without a significant installation space, thereby making a motor and a caliper body more compact.
In the present invention, since the great speed reduction ratio can be obtained, a great brake operating force can be obtained by a small torque of the motor. This means that the number of revolutions of a rotor of the motor becomes relatively high. Accordingly, if pad clearance becomes too great, since it takes a long time for starting the operation of the brake, it is more important that the pad clearance is always kept constant in comparison with the conventional electric disc brakes.
In this regard, conventionally, as a pad wear following mechanism capable of being applied to the electric disc brake, for example, There has been proposed a mechanism by which, in providing a coil spring, and a one-way clutch or a non-reversible screw, a wear following operation is effected by utilizing rotation of a rotor of an electric motor (for example, refer to Japanese Patent Laid-open No. 55-69337 and International Publication No. WO 99/02885).
Further, although various wear following mechanisms capable of being applied to the electric disc brake have been proposed, since an amount of adjustment through one step is small and since a manual operation must be effected when the piston is retracted, exchange of pads becomes troublesome and time-consuming.
Accordingly, another object of the present invention is to provide an electric disc brake having a pad wear adjusting mechanism in which an amount of adjustment through one step can be increased and a piston can be retracted by rotation of a motor.
To achieve the above objects, according to the present invention, There is provided an electric disc brake in which a caliper body is floatingly supported by a carrier secured to a non-rotating portion of a vehicle, and the caliper body is provided with a piston opposed to one of a pair of brake pads supported by the carrier on both sides of a disc rotor, an electric motor, and a motion converting mechanism for converting rotation of a rotor of the electric motor into a linear movement and for transmitting such movement to the piston. A differential speed reduction mechanism is disposed between the rotor of the electric motor and the motion converting mechanism.
In the present invention, the differential speed reduction mechanism may be interposed between the rotor and the motion converting mechanism along an axial direction of the rotor. In this case, a diameter of the electric motor can be made smaller.
Further, in the present invention, the motion converting mechanism may comprise two rotary members as constructural elements, and the differential speed reduction mechanism may be provided with a gear mechanism for generating a difference in number of revolutions between two rotary members.
In this case, when the motion converting mechanism comprises a ball-ramp mechanism, the two rotary members are constituted by two rotary discs opposed to each other with the interposition of balls, and, when the motion converting mechanism comprises a ball-screw mechanism, the two rotary members are constituted by a nut and a threaded shaft. For a gear mechanism for the former, a combination of two external gears provided in coaxial arrangement with the rotor of the electric motor, and internal gears provided on the two rotary discs of the ball-ramp mechanism, respectively, and meshed with the external gears independently is used. On the other hand, for a gear mechanism for the latter, a combination of two internal gears provided in coaxial arrangement with the rotor of the electric motor, and external gears provided on the nut and the threaded shaft of the ball screw mechanism, respectively, and meshed with the internal gears independently is used. In any case, by the combination of the external gears and the internal gears, the speed of the rotation of the electric motor can be decreased efficiently without excessive installation space.
In the present invention, the motion converting mechanism may comprise one rotary member and one non-rotary member as constructural elements, and the differential speed reduction mechanism may be provided with an eccentric shaft provided on the rotor of the electric motor, and a differential mechanism for transmitting the reduced rotation of the rotor to the rotary member in accordance with rotation of the eccentric shaft.
In this case, the motion converting mechanism may comprise a ball-ramp mechanism in which balls are interposed between the rotary disc and the fixed disc, and the differential mechanism may comprise an eccentric plate rotatably fitted onto the eccentric shaft provided on the rotor, an oldham""s mechanism for giving an orbital movement to the eccentric plate in accordance with rotation of the rotor, and a speed reduction device for rotating the rotary disc of the ball-ramp mechanism with a given or fixed rotational ratio with respect to the rotor in response to the orbital movement.
The electric disc brake according to the present invention having the ball-ramp mechanism may be combined with an improved brake pad wear following mechanism. That is to say, an electric disc brake based on one aspect of the present invention is characterized in that a rotor of the electric disc brake is connected to a first disc of a ball-ramp mechanism, a piston is threaded in a second disc via a threaded portion, a spring device is connected between the first and second discs, the piston is advanced or retracted by integrally rotating the first and second discs by a spring force of the spring device, and the piston is retracted or advanced by effecting relative rotation between the first and second discs in opposition to the spring force of the spring device.
With this arrangement, when the disc rotor is not urged by the brake pad, since a great load does not act on the threaded portion to reduce resistance of the threaded portion, upon the rotation of the rotor of the electric motor, the first and second discs are integrally rotated together via the spring force of the spring device, with the result that the piston is shifted by the threaded portion. On the other hand, when the disc rotor is urged by the brake pad, since a great load acts on the threaded portion to increase resistance of the threaded portion, upon the rotation of the rotor of the electric motor, the first and second discs are rotated relative to each other in opposition to the spring force of the spring device, with the result that the piston is driven by the ball-ramp mechanism.